Optimisation of bit patterned media for 1 Tb/in

Micromagnetic simulations were used to investigate the influence of patterned media geometry on the signal to noise ratio (SNR), adjacent track erasure and write margin for a target recording density of 1 Tb/in². For an ideal patterned medium the readback noise was a maximum when the read head was directly over the dots and a minimum at the transitions. The SNR improved for smaller dots due to the larger dot separation. However, the ideal media with the highest SNR were also the most susceptible to dispersions of dot size and position. Low temperature simulations suggest that large write margins are available; however, at room temperature the write margin can be much reduced. Increasing the rise time of the write head had a deleterious effect on the write margin and the write margin was zero for rise times of more than 0.45 ns. Nevertheless, error-free writing at 1 Tb/in² could be achieved using appropriate head geometries and material parameters.     

Preparation and recording characteristics of granular-type perpendicular magnetic recording media with thin intermediate layer

Granular-type media with thin Ru intermediate layer were prepared on a highly oriented high-B_s FeCo soft underlayer (SUL). A CoPt–TiO₂ recording layer on a Ru intermediate layer of only 2 nm had high-crystal orientation, high H_c of 6.5 kOe, and a high squareness ratio (SQ) of 0.99. The magnetic property of the SUL was also good. The recording performance was measured for the media with different Ru intermediate thicknesses by using a single-pole-type (SPT) head. The media had large reproduced output even for the Ru intermediate layer thickness of 2 nm.     

Perpendicular magnetic recording technology at 230 Gbit/in

A perpendicular recording system that allows areal densities beyond 200 Gbit/in² has been designed and tested to investigate the major challenges in perpendicular magnetic recording. The integrated write/read head has a trailing shield to improve the write head field gradient and a conventional CIP–GMR reader. The medium is a low-noise CoPtCr-based oxide medium with a CoTaZr soft underlayer. On track byte error rates at ∼ 50 Mb/s are better than 10⁻⁴ at ∼1000 kbpi. Using a 15% off-track criterion at 10⁻² byte error rate, track densities between 200–240 ktpi are realized, yielding areal densities of 210–233 Gbit/in². High-resolution magnetic force microscopy (hrMFM) has been employed to investigate the write characteristics of these heads with improved cross-track resolution. Using a quantitative analysis method, many parameters, such as transition curvature and transition width, are estimated from the hrMFM image. Significant transition curvature is found, which increases the width of the read head response to a transition, T₅₀, by 2–3 nm.These results give insights into the recording physics of perpendicular recording and in particular point out improvements required for achieving even higher areal densities.     

Looking back at perpendicular magnetic recording R&D in NEC

This paper reports perpendicular magnetic recording (PMR) circumstances and the results based on the reference published by NEC. The PMR flexible disks using Co-Cr-Ta films were investigated. Pass wear durability of Co-Cr-Ta film strongly depends on the base film surface roughness and Young's modulus values of Co-Cr-Ta films. Pass wear durability, more than 10 million passes, was confirmed under a high temperature (60 °C) and a high humidity (80% RH) condition, as well as a low temperature (5 °C) condition. The read/write characteristics for double-layered PMR media were examined by using a combination of a single-pole-type (SPT) write head and a magnetoresistive (MR) read head, and a conventional merged ring type inductive (ID) write head/MR read head. By differential equalization of the reproduced voltage, the bit error rate less than 10⁻⁶ at 3 Gb/in.² was obtained for the SPT/MR head combination. The antenna effect for PMR realization was analyzed using the merged ring type ID/MR head. In order to increase the magnetic circuit resistance from the ID head pole to the soft underlayer, we developed the sendust (FeAlSi) soft underlayer with low magnetic permeability. We confirmed that the recorded signal has better stability under the ID/MR head-loaded condition than the SPT head-loaded condition. These results show that there are the head and media solutions to realize PMR. We had confidence that we could use the basic structure of the commercial ID/MR head for the PMR head.     

Micromagnetic simulations of discrete track media

Discrete track media were simulated and compared with conventional, continuous media. The optimum write pole width was determined for discrete track media with 60 nm land/30 nm grooves, subject to the condition that there be no adjacent track erasure. Under optimal conditions discrete track media outperformed continuous media in terms of both on-track and off-track SNR. The performance difference became greater at higher linear densities, demonstrating the potential of discrete track media.     

CPP-GMR heads with a current screen layer for high areal density

We investigated the potential for high-areal-density recording in current-perpendicular-to-plane giant magnetoresistive (CPP-GMR) heads with a current screen layer. The current screen layer is a nano-oxide layer with confined current paths. We fabricated the current screen CPP-GMR heads with a narrow sensor width of 40–50 nm, a high MR ratio of 17%, and low-resistance-area (RA) product of 0.2 Ω μm². The fabricated heads showed a sufficiently high signal-to-noise ratio (SNR) of 30–40 dB. No extra noise, such as spin-torque noise, was observed. Linear density of 1360 kFCI from the head with the magnetic read width of 45 nm was obtained. Distribution of sensor resistance due to nano-hole area distribution in the current screen layer can be reduced with low-RA film. Spin-torque noise can be suppressed by reducing the current-induced field and controlling the shape anisotropy. Accordingly, the current screen CPP-GMR head is a promising candidate that has the potential for high-areal-density recording.     

Angular dependence of coercivity and remanence of Ni nanowire arrays and its relevance to magnetoviscosity

Ni nanowire arrays with varying wire dimensions (diameter d, length l) and center-to-center distances d_CC were synthesized by pulsed electrodeposition of Ni in porous Al templates. The magnetization-reversal behavior of the arrays was investigated by means of magnetometry for different angles θ between the wire axes and the applied magnetic field. The functional dependences of the characteristic parameters coercivity H_C(θ) and reduced remanence m_R/m_S(θ) exhibit a strong dependence on the wire dimensions and the center-to-center distance. For instance, for nanowire arrays with d=40 nm, d_CC=100 nm, and for θ=0°, the coercivity takes on a rather large value of μ₀H_C=85 mT and m_R/m_S≅94%; reducing d_CC to 30 nm and d to 17 nm results in μ₀H_C=49 mT and m_R/m_S≅57%, an observation which suggests an increasing magnetostatic interwire interaction at increased (d/d_CC)-ratio. The potential application of nanowires as the constituents of ferrofluids is discussed.     

Improvement of magnetic properties and read/write characteristics in SmCo5 perpendicular thin films

An SmCo₅ alloy is a promising candidate for ultra-high density magnetic recording media because of its strong uniaxial magnetocrystalline anisotropy, whose constant, K_u, is more than 1.1×10⁸ erg/cm³. Recently, we successfully obtained high perpendicular magnetic anisotropy for a sputter-deposited SmCo₅ thin film by introducing a Cu/Ti dual underlayer. However, it is necessary to improve magnetic properties and read/write (R/W) characteristics for applying SmCo₅ thin films to perpendicular magnetic recording media. In this study, we focused on reduction of magnetic domain size and change of a magnetization reversal process of SmCo₅ perpendicular magnetic thin films by introducing carbon (C) atoms into the constituent Cu underlayer. The magnetic domain size became small and the ratio of coercivity (H_c) against magnetic anisotropy (H_k) which is an index of the magnetization reversal process was increased by adding C atoms. We also evaluated the R/W characteristics of SmCo₅ double-layered media including C atoms. The medium noise was decreased and signal-to-noise ratio increased by introducing the C. The addition of C into the Cu underlayer is effective for changing the magnetization reversal process, reducing medium noise and increasing SNR.     

Influence of Pt thickness on magnetization reversal processes in (Pt/Co)3 multilayers with perpendicular anisotropy

We present a detailed study of the magnetization reversal in perpendicularly magnetized (Pt/Co)₃ multilayers with different values of the platinum interlayer thickness t_Pt. To study the magnetization reversal in our samples we combined measurements of relaxation curves with the direct visualization of domain structures. Magnetization reversal was dominated by domain wall propagation for t_Pt=1 nm and by domain nucleation for t_Pt=0.2 nm, while a mixed process was observed for t_Pt=0.8 nm. We interpret our results within the framework of a model of thermally activated reversal where a distribution of activation energy barriers is taken into account. The reversal process was correlated with the energy barrier distribution.     

Recording simulation beyond 2 Tbit/in on bit-patterned media with shielded planar head

A simple technique for bit-patterned media was proposed to increase achievable areal recording densities beyond 2 Tbit/in². Introduction of longitudinal magnetic anisotropy to the media indicated reduced effect of magnetostatic interaction between the dots. Recording simulation with a shielded planar pole head exhibited increased write shift margins in both down and cross track directions compared with that of the perpendicular anisotropy media. It was suggested that recording of an areal density of 2.5 Tbit/in² would be realized with a down and cross track margins of 3.5 and 4.0 nm, respectively. Better recording performance at high areal densities is expected if suitable head could be designed.     

Influence of patterning fluctuation on read/write characteristics in discrete track and bit patterned media

The read/write characteristics of non-patterned media (NPM), discrete track media (DTM), and bit patterned media (BPM) are examined by modeling the magnetization distribution of NPM and patterning fluctuation of DTM. By comparing spin-stand measurement with calculation, the magnetization distribution of NPM was well characterized with a new Voronoi cell magnetic cluster model, in which the cluster size at the track edge, 〈D_edge〉, was larger than that at the track center, 〈D_center〉 by a factor of two. Based on an analysis of patterning fluctuations seen in SEM images of DTM, line-edge roughness (LER) was modeled as a long-wavelength center-line roughness (CLR) plus a short-wavelength line-width roughness (LWR). It was confirmed that the standard deviation of the patterning fluctuation was much smaller than that of the magnetic fluctuation for NPM. This allowed DTM to achieve higher off-track performance than NPM. By examining the 747 curves, it was revealed that DTM could have an advantage in track-density of up to approximately 25% assuming patterning fluctuations can be well controlled at high track density. In BPM, fabricating accurate dots is essential. The relationship between dot defect rate and patterning fluctuation was examined, and the maximum allowable standard deviation of LER was derived as 2 nm for achieving 1 Tbspi.     

The control of the diameter of the nanorods prepared by dc reactive magnetron sputtering and the applications for DSSC

The TiO₂ nanorod arrays, with about 1.8 μm lengths, have been deposited on ITO substrates by dc reactive magnetron sputtering at different target-substrate distances. The average diameter of these nanorods can be modified from about 45 to 85 nm by adjusting the target-substrate distance from 90 to 50 mm. These nanorods are highly ordered and perpendicular to the substrate. Both XRD and Raman measurements show that the nanorods prepared at different target-substrate distances have only an anatase TiO₂ phase. The nanorods prepared at the target-substrate distance less than 80 mm have a preferred orientation along the (2 2 0) direction. However, this preferred orientation disappears as the target-substrate distance is more than 80 mm. These TiO₂ nanorods have been used as the electrodes for dye-sensitized solar cells (DSSCs). The highest conversion efficiency, about 4.78%, has been achieved for TiO₂ nanorods prepared at 80 mm target-substrate distance.     

Design and recording simulation of 1 Tbit/in patterned media

Design of patterned media for an areal density of 1 Tbit/in² with thermal stability is presented based on perpendicular M–H loops of the media. Required perpendicular magnetic anisotropy was estimated to be achieved with known materials. However, it is indicated that magnetostatic interaction between the dots becomes a limiting factor for achieving higher densities. Recording simulation using a Karlqvist pole head on the designed media exhibited possibility of the recording of 1 Tbit/in². Shift margin of the write head in the cross-track direction was found to be increased with elongated dots in the down-track direction. Recording simulation with an FEM-analyzed field of a side-shielded multi-surface pole head exhibited successful recording with increased cross-track shift margins as well as the effect of the elongated dot shape.     

Low-temperature growth of ZnO nanorods on PET fabrics with two-step hydrothermal method

An effective low-temperature growth method to fabricate hexagonally oriented ZnO nanorod arrays onto PET fabrics is reported. The effect of substrate pre-treatment and C₆H₁₂N₄ concentration on the structure of ZnO nanorod arrays were investigated in details by X-ray diffraction (XRD), FE-SEM and ultraviolet protection factor (UPF). The results show that substrate pre-treatment, C₆H₁₂N₄ concentration indeed have great influence on the growth of ZnO nanorod arrays. It is indispensable to introduce a ZnO seed layer on the substrate and under growth condition of n(C₆H₁₂N₄):n[Zn(NO₃)₂] = 1:1, T = 90 °C, t = 3 h, the well-aligned ZnO nanorod arrays with 40-50 nm in diameter and 300-400 nm in length were achieved on the pre-treated PET fabrics. The ZnO nanorods grown on PET fabrics possessed an ultrahigh ultraviolet protection factor of 480.52 in this study, indicating an excellent protection against ultraviolet radiation in comparison with the untreated PET fabrics.     

Performance comparison of pre-, post- and symmetrical-dispersion compensation techniques using DCF on 40 Gbps OTDM system for different fibre standards

Various factors like polarization mode dispersion, non-linear effects, Kerr effect, second and third order dispersion impose limit on the performance (transmission distance, pulse broadening) of Optical Time Division Multiplexed (OTDM) transmission system. In this paper, the performance comparison of 40 Gbps OTDM transmission system with pre-, post-, and symmetrical-dispersion compensation techniques for different fibre standards has been carried out. It has been observed that for pre compensation, ITU 655 [D = 3.78 ps/nm/km] came to be best suitable fibre with the dispersion compensating fibre length of 2.36 km for maximum reach of 3000 km. For post- and symmetrical-compensation, the behaviour of alcatel [D = 8 ps/nm/km] is almost similar with maximum transmission distance of 3200 and 3050 km respectively.     

Playback performance and microstructure of sputtered tape media formed by facing targets sputtering

Sputtered tape media of a CoPtCr–SiO₂ magnetic layer with a Ru underlayer was fabricated on a thin Aramid film by a facing targets sputtering (FTS) system at room temperature with no cooling. Transition electron microscope (TEM) images show columnar epitaxial growth of hcp-CoPtCr(1 0 0) plane on hcp-Ru(1 0 0). Average grain diameter of 10.2 nm with dispersion of 20.2% was obtained from TEM images. Enrichment of Co and Pt inside grains and segregation of Cr and Si to boundary were confirmed by point energy dispersive spectroscopy (EDS) measurements. Playback performance test on sputtered sample indicated that SNR is higher and PW50 value is lower than that of commercial coated tapes. These good playback properties could come from fine and isolated grain structure of magnetic layer of sputtered sample, as observed by TEM.     

Demonstration of ultra-high-resolution MFM images using Co90Fe10-coated CNT probes

We demonstrate ultra-high-resolution magnetic force microscopy images of perpendicular magnetic storage media using carbon nanotube probes coated by ferromagnetic Co₉₀Fe₁₀ films (20, 30, 40, and 50 nm). By optimizing ferromagnetic film thickness (effective tip diameter), we obtained best magnetic domain image with an 40 nm-Co₉₀Fe₁₀-coated tip (50 nm tip diameter) about a lateral detect density of 1200 k flux per inch on perpendicular magnetic storage medium, one of the highest resolutions in MFM imaging reported for this material system and structure. The observed dependence of tip dimension on signal contrast and image resolution was successfully explained by a theoretical analysis indicating that the signal contrast, along with the physical probe-tip dimension, should be taken into account to design magnetic probes tips for high-resolution magnetic force microscopy.     

Dual behaviors of magnetic CoxFe1−x (0≤x≤1) nanowires embedded in nanoporous with different diameters

Co_xFe_1−x nanowire arrays with various diameters and different composition were fabricated by ac electrodeposition using porous alumina template. Coercivity along the easy axis reaches to a maximum at 2330 Oe, for Co_xFe_1−x nanowires containing about 40 at% Co. The crystalline structure of the nanowires was concentration-independent and shows a bcc structure. The critical diameter for transition from coherent rotation to curling mode is 35 nm for CoFe containing less than 40 at% Co while it is 30 nm for those with more than 40 at% Co. Optimizing the magnetic properties of CoFe with different Co content was seen to be dependent on the diameter of nanowires. For 25 nm diameter, the optimum was shown below 50 at% Co while it was seen above 50 at% for nanowires with 50 nm diameter. The angular dependence of the coercivity with nanowires diameter were also studied.     

L10-ordered high coercivity (FePt)Ag-C granular thin films for perpendicular recording

We report (FePt)Ag-C granular thin films for potential applications to ultrahigh density perpendicular recording media, that were processed by co-sputtering FePt, Ag, and C targets on MgO underlayer deposited on thermally oxidized Si substrates. (FePt)_1−xAg_x-yvol%C (0<x<0.2, 0<y<50) films were fabricated on oxidized silicon substrates with a 10 nm MgO interlayer at 450^oC. We found that the Ag additions improved the L1₀ ordering and the granular structure of the FePt-C films with the perpendicular coercivity ranging from 26 to 37 kOe for the particle size of 5-8 nm. The (FePt)_0.9Ag_0.1-50vol%C film showed the optimal magnetic properties as well as an appropriate granular morphology for recording media, i.e., average grain size of D_av=6.1 nm with the standard deviation of 1.8 nm.     

The epitaxial growth of BaSnO3 buffer layer on IBAD-MgO template and its effects on GdBa2Cu3O7−δ film: A preliminary study

We report a successful fabrication of c-axis oriented GdBa₂Cu₃O_7−δ (GdBCO) films on the BaSnO₃ (BSO) buffer layers on ion-beam assisted deposition (IBAD)-MgO template by pulsed-laser deposition (PLD). The (0 0 l) growth and in-plane textures of BSO buffer layers were found sensitive to the substrate temperature (T_s). With increasing the BSO layer thickness up to ∼165 nm, in-plane texture (Δ? ∼ 6.2°) of BSO layers was almost unaltered while completely c-axis oriented BSO layers were obtainable from samples with the thickness below ∼45 nm. On the BSO buffer layers showing in-plane texture of 6.2° and RMS surface roughness of ∼8.6 nm, GdBCO films were deposited at 780–800 °C. All GdBCO films exhibited Δ? values of 4.6–4.7°, T_c,zero of ∼91 K, and critical current density (J_c) over 1 MA/cm² at 77 K in a self-field. The highest J_c value of 1.82 MA/cm² (I_c of 51 A/cm-width) was achieved from the GdBCO film deposited at T_s of 790 °C. These results support that BSO can be a promising buffer layer on the IBAD-MgO template for obtaining high-J_c GdBCO coated conductors.